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Using Soils to Reconstruct Mid-continental Climatic Change
Christoph E. Geiss, Trinity College
Collaborators and Students:C. William Zanner, Univ. of Nebraska, LincolnSubir K. Banerjee, Univ. of MinnesotaJames Bisbee, Daniel Scollan, Trinity CollegeJoanna Minott, Mt. Holyoke College
Great Plains Region
• intensively farmed agricultural region• western part dependent on irrigation• very few good records of paleoclimatic change• large parts covered by wind-blown dust (loess)
Long-Term Plan• establish transfer
function between modern climate and soil properties
• invert transfer function and apply to buried soils (paleosols)
• reconstruct paleoclimate for certain time slices over last 130,000 years
• low temporal, but high spatial resolution
Site Selection
• modern soils
• loessic substrate
• stable upland positions
• transsect to capture climate gradient
most soil forming factors are held constant, except for climate and biota (which are assumed to be controlled by climate)
Site Selection
• initial sites: public lands
• second fieldseason: cemetaries (undisturbed by agriculture, set aside often prior to settlement)
Useful Parameters
• In-situ profile description
• Color (Munsell and spectrophotometer)
• Chemistry (org. matter, carbonates, Fe, Mn)
• Magnetic enhancement of upper soil horizons
Chinese Loess Plateau
• Modern soil and paleosols are more magnetic than loess
• Magnetic enhancement of modern soils reflects modern precipitation gradient
• Paleoprecipitation proxy?
Xifeng loess – paleosol profilemodified fromKukla et al, Geology, 16, 811-814, 1988
Questions
• What causes magnetic enhancement ?
• Is magnetic signal preserved after burial?
• Is magnetic enhancement a universal proxy ?
• Can we use it to reconstruct paleoclimate for central United States?
Some Potential Processes of Magnetic Enhancement• Depletion of non-magnetic particles
(lessivage)
• Reduction of weakly magnetic minerals to magnetite / maghemite
• Neoformation of Fe-oxides / Fe-oxyhydroxides
• Systematic changes in parent material ?
Magnetic Methods
Want to characterize:
• Abundance• Particle-size distribution• Mineralogy
indirect (magnetic) methods: fast, (mostly) sensitive, cost-effective
Concentration of Ferrimagnetic Minerals• Magnetic susceptibility
• Isothermal Remanent Magnetization (IRM)
• Anhysteretic Remanent Magnetization (ARM)
• and a few others
Example: Site 4G-99A
• Located in NE Nebraska
• Sampled in 1999 using Giddings corer
• Subsampled into plastic boxes in 2000
• Analyzed in 2000 (REU project) and 2003
Characterization of Magnetic Grain-size
• grainsize characterized by domain state
– multi domain MD (< 10 μm)– single domain SD (0.01 – 0.1 μm)– superparamagnetic SP ( < 0.01 μm)
domain state affects magnetic behavior of mineral grains
Grain-size Dependent Parameters
• many parameters concentration and grainsize dependent
• normalized parameters– ARM / IRM : fine SD particles– susceptibility / IRM : super fine SP particles
• Frequency dependent susceptibility (SP)
Normalized Parameters
• IRM, ARM both concentration and grain-size dependent
• Ratio of ARM/IRM (concentration indep.) mostly proxy for small single-domain (SD) grains,(d ≈ 0.01 – 0.1 µm)
Magnetic Mineralogy
• magnetic minerals occur – in low concentrations (< 1 %)– in poorly crystalline states→ hard to characterize using XRD, Mössbauer
etc.
• magnetic ordering and phase transitions• magnetic coercivity measurements
but: magnetization of magnetite >> magnetization of goethite, hematite
IRM-Acquisition Curves
• describes how easy mineral is to magnetize
• magnetite = magnetically soft, saturates in low fields
• hematite, goethite = magnetically hard, probably impossible to saturate
after Butler, J. Geophys. Res., 87, 7843-7852 , 1982
Coercivity the Cheapo Way
• S-ratio: gives relative abundance of hard/soft minerals
• Hard IRM (HIRM): gives absolute abundance of hard/soft minerals
J300
Jsat
modified from:Butler, J. Geophys. Res., 87, 7843-7852 , 1982
4G-99A magnetic summary
• upper soil horizons are enhanced in magnetic minerals
• concentration increases
• grainsize decreases
• pedogenic component is mixture of magnetite and goethite / hematite
Cause for Magnetic Enhancement
• concentration of Fe slightly decreases in enhanced horizons
• weathering of Fe-bearing minerals and neoformation of poorly crystalline magnetite and goethite/hematite
• microbially mediated?
• Midwestern modern soils are magnetically enhanced
• Climatic influence seen best in parameters that are biased towards small particles
• Magnetic enhancement due to neoformation of magnetite and magnetically hard minerals such as goethite or hematite
• Neoformation likely aided by microbial activity
Some Preliminary Conclusions